High-throughput, in vivo screening platform for modulators of Apolipoprotein B

ABSTRACT

We describe a high-throughput, phenotypic screening method for one or more modulator(s) of Apolipoprotein B (ApoB) in larval zebrafish. The modulator(s) may be enhancers or inhibitors of ApoB expression. This represents a remarkable opportunity to investigate drug targets in every cell and tissue type of a whole animal without bias, thus maximizing the likelihood of identifying viable pre-therapeutic leads for compounds or biologics in a subject (e.g., human).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 62/347,178, filed Jun. 8, 2017; the content of which is incorporated by reference in its entirety.

STATEMENT REGARDING FEDERAL GOVERNMENT SUPPORT

This invention was made at least in part with federal funds under NIH research project grants R01-DK111620 and R01-DK093399. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

Elevated serum Apolipoprotein B (ApoB) may represent a unifying risk factor underlying many of the world's most prevalent metabolic diseases. ApoB levels independently predict incidence of diabetes, metabolic syndrome, and cardiovascular disease even after adjusting for confounding variables. While it may seem implausible that a single disease marker could be implicated in such a broad range of diseases, the frequent clustering of these metabolic abnormalities is suggestive of shared causal risk factors. Also, the characterized roles of ApoB in lipid transport, endoplasmic reticulum stress, inflammation, and atherogenesis provide straightforward mechanistic connections to the etiologies of each linked metabolic disease. ApoB therefore represents a highly promising therapeutic target to combat the growing global burden of metabolic disease. Methods identifying agents that modulate the expression of ApoB must be developed to produce new pharmaceutical agents that prevent and/or treat these diseases.

SUMMARY OF THE INVENTION

We describe a high-throughput phenotypic screening method for one or more modulator(s) of Apolipoprotein B (ApoB) in larval zebrafish. In vivo screening avoids the disadvantages of cell-free and in vitro systems. This represents a remarkable opportunity to investigate drug targets in every cell and tissue type of a whole animal without bias, thus maximizing the likelihood of identifying viable pre-therapeutic leads for compounds or biologics in a subject (e.g., human).

One embodiment is a method of high-throughput, in vivo screening comprising the steps of: applying an agent to a zebrafish expressing an ApoB-reporter fusion protein, monitoring the reporter activity, comparing the reporter activity to a second reporter activity of a reference, and identifying a modulator of ApoB. It is preferred that an ApoB-reporter fusion protein is expressed from an ApoBb.1 locus-reporter gene, wherein the reporter is a luciferase. The methods of the present invention may be used to identify one or more modulator(s) of ApoB, which enhance ApoB expression when the reporter activity is greater than the second reporter activity of the reference. Moreover they may be used to identify one or more modulator(s) of ApoB, which inhibit expression of ApoB when the reporter activity is less than the second reporter activity of the reference. Suitable agents or entities used in the present invention include chemicals (e.g., low molecular weight); natural products; nucleic acids (e.g., RNAi, antisense, aptamer); proteins (e.g., agonist or antagonist based on native ligand of receptor, soluble receptor, antibody); and peptidomimetics. An example of a suitable fusion protein that is stably expressed by zebrafish of the present invention includes ApoB-NanoLuc® luciferase fusion protein expressed from a DNA sequence of SEQ ID NO: 3. An example of a protein sequence of a suitable fusion protein that is stably expressed by a zebrafish of the present invention is SEQ ID NO: 2.

Another embodiment of the present invention is a zebrafish comprising an ApoBb.1 locus-reporter fusion gene wherein a preferred reporter is luciferase such as an ApoBb.1 locus-NanoLuc® luciferase fusion gene of DNA sequence of SEQ ID NO: 3, and this zebrafish of the present invention may contain additional reporters whereby a genomic ubiquitous promoter drives expression of a firefly luciferase gene, a mCherry fluorescent reporter, or a combination thereof.

Another embodiment of the present invention is a method of high-throughput in vivo screening to identify a modulator of ApoB comprising the steps of: applying agents to zebrafish larvae expressing an ApoB-reporter fusion protein gene and a second reporter protein; monitoring the ApoB-reporter fusion protein and the second reporter protein activities; and comparing the reporter activities to the third reporter activities of a reference. It is preferred that the ApoB-reporter fusion protein is an ApoB-luciferase fusion protein expressed from an ApoBb.1 locus-NanoLuc® luciferase gene fusion. When monitoring protein activities it helps to sonicating the zebrafish larvae before measuring reporter activity. One suitable method of measuring reporter activity, such as an ApoB-fusion protein and/or of a second reporter protein, is by a high content screening (HCS) microscopy platform. A suitable second reporter is firefly luciferase.

The term “activity” refers to the ability of a gene or its product to perform a function, such as luciferase being able to catalyze a reaction to produce light. For example, an optical signal that can be quantified (e.g., light) may be preferred.

The term “ApoBb.1-NanoLuc® luciferase gene” is a DNA sequence comprising an ApoB promoter, ApoB gene or functional part thereof, fused to a NanoLuc® luciferase gene or functional part thereof as described below. FIG. 1 shows an example of such a gene fusion and corresponding protein fusion. The genomic ApoBb.1-NanoLuc® luciferase gene has a nucleotide sequence including introns and portions of the UTR that is at the native genomic locus of zebrafish and therefore contains all introns, untranslated regions, and cis regulatory elements of the native ApoBb.1 gene (SEQ ID NO: 3). The coding sequence (i.e., cDNA of spliced mRNA that will be translated to produce an ApoBb.1-NanoLuc® luciferase fusion protein) is SEQ ID NO: 1.

The term “ApoBb.1-NanoLuc® luciferase protein” refers to a protein having both ApoB activity and luciferase activity. An example of the amino acid sequence of such a protein is SEQ ID NO: 2.

The term “express” refers to the ability of a gene to express the gene product including for example its corresponding mRNA or protein sequence (s).

The term “luciferase” refers to an enzyme that produces bioluminescence such as NanoLuc® luciferase or firefly luciferase as examples. The term “NanoLuc” is the tradename of a luciferase that is specific to a NanoLuc® enzyme developed by Promega, referred to as “NanoLuc® luciferase.”

The term “reference” refers to a standard or control conditions such as a sample (e.g., zebrafish that does not contain the reporter gene or express its reporter gene product, human cells), a subject that is free, or substantially free, of agent or entity.

The term “reporter gene” or “reporter” refers to a gene attached to a regulatory sequence of another gene of interest in bacteria, cell culture, animals, or plants. Certain genes are chosen as reporters because the characteristics they confer on organisms expressing them are easily identified (e.g., present or absent) and measured (e.g., quantification), or because they are selectable markers. Reporter genes are often used as an indication of whether a certain gene has been taken up by or expressed in the cell or organism population. Commonly used reporter genes that induce visually identifiable characteristics (e.g. optical signal measured by intensity or polarization) usually involve fluorescent and luminescent proteins. Examples include the gene that encodes jellyfish green fluorescent protein (GFP), which causes cells that express it to glow green under blue light, the enzyme luciferase, which catalyzes a reaction with luciferin to produce light, and the red fluorescent protein from the dsRed gene. Modified fluorescent and luminescent genes produce light having distinguishable maximal wavelengths, thereby enabling simultaneous identification and measurement of multiple different reporters.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of the generation of a genomic ApoB-NanoLuc® luciferase fusion protein. Specially designed TALEN nucleases introduce a double-stranded break just upstream of the stop codon of the Zebrafish ApoBb.1 gene. This lesion is repaired through homology-directed repair using the homology arms of the donor plasmid as a template, resulting in formation of the desired fusion construct between ApoB and NanoLuc® luciferase.

FIGS. 2A and 2B show lipoprotein size distributions detected by (FIG. 2A) in-gel luciferase assay and (FIG. 2B) fluorescent scan of BODIPY-stained adult zebrafish serum.

FIG. 3 shows signal and background measurements for the primary and counter-screen luciferase assays showing signal-to-background ratios of >10,000:1.

FIGS. 4A and 4B show dose-responses of ApoB reporter signal to established pharmacological and physiological modulators. (FIG. 4A) Larval zebrafish were treated with various concentrations of lomitapide (an inhibitor of MTP) for 36 hours. (FIG. 4B) Larval zebrafish were fed a high-fat meal and then fasted for 0, 1, or 2 days.

FIGS. 5A and 5B illustrate visualization of fluorescent lipid transport in larvae treated with (FIG. 5A) vehicle control and (FIG. 5B) lomitapide. FIG. 5C shows PCA analysis using the finotyper program to separate lomitapide treated (dashed) from vehicle treated controls (solid).

FIGS. 6A and 6B show LDL-size distributions in (FIG. 6A) normal in untreated controls but (FIG. 6B) particle size and number are greatly reduced in the lomitapide-treated sample.

FIGS. 7A to 7C show ApoB-NanoLuc® luciferase reporter assay performance. (FIG. 7A) Z′-factor (Z′) is shown for genetic negative and positive controls; pharmacological negative and positive (lomitapide) controls. (FIG. 7B) For humans treated with the strongest ApoB-lowering therapy available, Z′ is shown for pharmacological negative and positive (lomitapide) controls; pharmacological negative and positive (a PCSK9 antibody) controls. (FIG. 7C) Z′ is shown for genetic negative and positive controls; pharmacological negative and positive controls.

FIGS. 8A and 8B show validation of the screening assay protocol. (FIG. 8A) Compounds implicated in the regulation of lipid and lipoprotein metabolism are listed with their mechanism of action. (FIG. 8B) Lomitapide dose-responses. The hit selection cutoff is set to −2.7 (dashed).

FIGS. 9A and 9B show the size distribution and localization of ApoB-containing particles. (FIG. 9A) Lipoproteins of wild-type zebrafish were electrophoretically separated on a polyacrylamide gel, then detected by immersion in luciferase substrate. Mutant zebrafish lacking the ApoC2 gene were unable to lipolyze chylomicrons (CM) and very-low density lipoprotein (VLDL) particles into their smaller counterparts, intermediate density lipoproteins (IDL) and low-density lipoproteins (LDL). Conversely, zebrafish treated with lomitapide did not have large lipoprotein particles. (FIG. 9B) Immersion of transgenic zebrafish in luciferase substrate enabled visualization of ApoB-NanoLuc® luciferase within larvae.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION

An in vivo high-throughput screening assay for modulators of ApoB in the larval zebrafish has been developed. For this purpose we have engineered the first ever transgenic zebrafish carrying a luciferase reporter fused to ApoB, which enables rapid and sensitive reporter quantification in 96-well plate format. We have shown that (i) the luciferase reporter does not interfere with ApoB function in vivo, (ii) the reporter shows the expected responses to physiological and pharmacological stimuli, and that (iii) the chemiluminescence assay used here shows substantial improvement in signal to noise and assay performance over any published large-scale screen in zebrafish.

The high-throughput screening (HTS) assay of the present invention has optimized the developmental stage of larvae used in the screen, (ii) modification of the robotics platform and optimization of buffer and treatment conditions to incorporate a homogenization step, and (iii) breeding of homozygous reporter lines for use in the primary screen. The high-throughput screen of the present invention screens the JHDL collection of 3,040 compounds utilizing a protocol for high-throughput screening in zebrafish called ARQiv-HTS (automated reporter quantification in vivo coupled to high-throughput screening robotics), with slight modifications to detect a chemiluminescent rather than a fluorescent readout. This quantitative HTS approach dispenses various dilutions of test compounds into wells containing individual larvae, which are subsequently quantified for ApoB levels using a plate reader. A counter-screening reporter has also been established that measures in parallel (dual-luciferase assay), which is used to exclude compounds that non-specifically reduce ApoB levels (e.g. cytotoxic and developmental delay responses).

An advantage of the JHDL is that many of the drugs have well-characterized targets, meaning that each hit implicates a corresponding target pathway as a regulator of ApoB. Putative pathways can be easily validated through treatment with known chemical probes to the putative target pathways. Modulation of ApoB is also likely to result in additional metabolic improvements, which can be assessed through a battery of secondary screens. Established fluorescent reporters of lipid transport, insulin signaling, atherogenesis, and inflammation can be assessed for responsiveness to lead compounds using high-content automated imaging, and resulting images are quantified using specialized statistical image analysis software. We have also developed a novel assay for monitoring lipoprotein size distribution that is included as an additional metabolic phenotype in the secondary screening process.

The luciferase used in the primary screening assay is the engineered NanoLuc® enzyme. This luciferase reporter is 100-fold brighter than the naturally occurring firefly or renilla luciferase enzymes. This increase in signal output provides a robust signal readout for the primary screening assay as well as the in-gel luciferase assay to detect lipoprotein size distribution for secondary hit validation screen. The NanoLuc® luciferase reporter is also significantly smaller than other frequently used fluorescent or bioluminescent reporters, reducing the likelihood that the tag disrupts native protein function.

Creation of NanoLuc® Luciferase-ApoB Fusion Knock-in Transgenic Line

ApoB has proven a difficult protein to study using traditional cloning and genome integration techniques due to its enormous size (approximately 14 kb of coding sequence, corresponding to a 540 kDa protein). Our early adoption of the recently developed precise genome engineering approaches in zebrafish have enabled us to develop the first fusion protein of ApoB ever reported, and one of the first endogenously tagged genes in the zebrafish genome.

Novel Quantitative Image Analysis Pipeline

The small size and optical clarity of the larval zebrafish has led to development of fluorescent reporters of numerous processes, as well as high-content screening (HCS) microscopy platforms capable of imaging them. For example, the VAST bioimaging platform (see the link WWW dot UNIONBIO dot COM slash VAST slash) provides unprecedented consistency and throughput for the imaging zebrafish and enables automated HCS. Our lab has developed improved methods for quantitative image analysis from whole-organism screening using principle component analysis (PCA) to quantify and cluster images into statistically different groups.

Use of the Larval Zebrafish in Whole-Organism Drug Screens In Vivo

The larval zebrafish model system provides the opportunity to study complex vertebrate disease phenotypes in the context of a highly tractable experimental system. Larval zebrafish are small, optically clear, develop rapidly, are easily bred in large numbers, can be consistently dosed with drugs placed directly in tank water, and are nourished by the maternally deposited yolk until about 5 days post-fertilization (dpf), making them ideal vertebrate systems for drug screens as well as whole-organism fluorescent imaging. Additionally, zebrafish have a proven track record in modeling human disease phenotypes, including several metabolic disease models such as diabetes, cardiovascular disease, and fatty liver disease. Thus, zebrafish have the potential to improve drug discovery by facilitating high-throughput phenotypic screening directly in living disease models.

Characterization of ApoB Paralogs in Zebrafish

As a first step in developing zebrafish as a model for studying the biology of ApoB, we have extensively characterized the phylogenetic history and expression pattern of the ApoB paralogs in zebrafish. In contrast to mammals, which have a single ApoB gene present in the genome, zebrafish possess 3 distinct paralogous ApoB genes named ApoBa, ApoBb.1, and ApoBb.2. Characterization of the evolutionary history (through phylogenetic and syntenic analysis), tissue specific expression pattern (through in-situ hybridization), and expression level of each of these paralogs (through RNA-seq and mass spectrometry) led the investigators to identify the ApoBb.1 gene as the predominant isoform of ApoB in zebrafish, individually accounting for approximately 95% of the ApoB mRNA and Protein expressed in the larval zebrafish. When ApoB is referred to in the application without the ApoBb.1 suffix, it is implied that this refers to the mRNA and protein derived from the ApoBb.1 locus in the zebrafish genome, rather than the ApoBa or ApoBb.2 genes or some combination of these genes.

Precise Fusion of NanoLuc® Luciferase to ApoBb.1 Locus

To overcome the difficulties of cloning the large ApoB gene and circumvent the complications of introducing transgenes into the genome, TALEN-mediated genome engineering was used to precisely integrate the NanoLuc® luciferase reporter as a fusion protein to the endogenous ApoBb.1 gene in zebrafish (see Shin et al. Development 141:3807-3818, 2014; Zu et al. Nature Methods 10:329-331, 2013; Neff et al. BMC Bioinformatics 14:1, 2013; Bedell et al. Nature 491:114-118, 2012). Co-injection of TALEN mRNA targeted just upstream of the stop codon of ApoBb.1 and a donor construct containing the NanoLuc® luciferase coding sequence flanked by approximately 500 bp homology arms was used to generate mosaic founders. Putative integrants were raised to sexual maturity and outcrossed to wild-type (AB*) larvae, and the resulting progeny were screened for heterozygous carriers of the desired transgene. Sequencing of the target locus confirmed that the integration event was error-free and in-frame, and protein expression was verified through chemiluminescent detection of the reporter in transgenic larvae.

Validation of Wild-Type Function of Tagged ApoBb.1

To evaluate whether introduction of the luciferase tag disrupted protein function, individual larvae carrying the reporter construct were homogenized and lipoprotein classes were separated on a 3% PAGE gel (see Singh et al. Lipids in Health and Disease 7:47, 2008). An in-gel luciferase assay was then performed to detect the NanoLuc® luciferase reporter present in the gel (FIG. 2A), and compared to the results obtained from adult zebrafish serum that was stained using lipophilic fluorescent dye (FIG. 2B). Encouragingly, essentially all of the signal was present in the expected VLDL, IDL, and LDL portions of the gel, which is consistent with the primary role of ApoB as an obligate structural component of lipid-rich lipoproteins. Additionally, this method has sufficient sensitivity and resolution to detect LDL subclasses, which are seen as deviations from the normal distribution in the plot profile. This indicates that the ApoB-luciferase fusion is lipidated, secreted, and processed in the pattern expected of native ApoB protein. Additionally, signal from the ApoB-luciferase fusion is responsive to known chemical and physiological modulators of ApoB (MTP inhibition and fasting).

Introduction of Counter-Screen Reporter

The primary screen seeks to identify compounds that lower ApoB levels, which is likely to identify numerous false positives that are cytotoxic or inhibit fundamental cellular processes such as transcription or translation. The inclusion of a counter-screen helps identify compounds that have strong effects on the target phenotype without causing major disruptions in other cellular processes. The pd75 zebrafish line carries a ubiquitous promoter driving firefly luciferase as well as the mCherry fluorescent reporter, which can be used to detect developmental delay or cytotoxicity in any organ. This line has been crossed to the ApoB reporter to generate double-transgenic zebrafish carrying both a ubiquitous firefly reporter and the ApoB reporter.

Independent Measurement of Multiple Luciferase Reporters in a Single Reaction

The Nano-Glo® dual luciferase assay provides the means to independently measure firefly and NanoLuc® luciferase in a single reaction enabling parallel measurement of the primary screen reaction and counter-screen reaction in a high-throughput compatible add-and-read format. Consistent measurement of luciferase activity requires homogenous sample inputs, so we have developed homogenization methods using a microplate sonicator that exceeds the throughput capacity of the plate reader.

Excellent Signal to Noise and Statistical Assay Performance

Signal to noise ratio: Luciferase reporters are essentially free of background noise, which has resulted in signal to background ratios between 100- and 2,000-fold higher than the best analogous fluorescent assays reported in zebrafish (FIG. 3).

Statistical Performance:

The most commonly used measure of HTS assay quality is the Z-factor (Z′). The Z′ for the primary assay is 0.67 for the primary screen and 0.76 for the secondary screen using the signal to background ratios shown in FIG. 2, reflecting an “excellent” high-throughput assay that is well above the threshold of 0.5 for an HTS-ready assay.

Sample-Size and Hit Cutoff Calculation:

Although Z′ is the most widely adopted statistical test to evaluate an assay for HTS, use of the strictly standardized mean difference (SSMD) is more appropriate for quality assessment and hit selection for drug screening in vivo as it accounts for unequal variance and outliers. To perform this statistical assessment on our assay, a titration test of lomitapide (the positive control drug treatment compound) was performed at 25, 10, 4, and 1.6 uM concentrations. This analysis determined that a sample size of 16 would be sufficient to detect compounds producing a small (25%) effect size relative to lomitapide control (Type I/II error rates of 0.01 and 0.001, respectively; Zα was 2.58; Zβ was 3.09). Using a sample size of 16, we then calculated a predicted SSMD score ‘hit’ cutoff, using bootstrapping with replacement on the lomitapide data, for detecting a 25% effect size. This analysis determined a score of >3.2 (or >3.6 for log transformed data) can be used to flag primary screen hits.

Proof of Principle Using Known Physiological and Pharmacological Modulators of ApoB

Known modulators of ApoB can be used as a proof of principle to demonstrate the sensitivity of the screen as well as the validity of the experimental model. Of the two existing inhibitors of ApoB, lomitapide is the most appropriate for use as a proof of principle because it is a small molecule. Conversely, mipomersen is an injectable antisense biologic that is not suitable for comparison to small-molecule inhibitors and cannot be used a high-throughput manner. Larval zebrafish were treated with serial dilutions of the lomitapide for 36 hours from 2.5 to 4 dpf. Lomitapide treatment results in a dose-dependent reduction in reporter activity relative to vehicle-treated controls (FIG. 4A).

Given the paucity of pharmacological modulators of ApoB, we sought to further validate our model by assessing responsiveness of the reporter to known physiological perturbations. ApoB levels are elevated after feeding as dietary lipid is packaged into ApoB-containing lipoproteins and transported throughout the body, and are therefore inversely correlated to time since their last feed. Zebrafish were fed a high fat diet on 6 dpf, and either re-fed or fasted for 1 or 2 days. We observed the expected inverse relationship between the reporter signal intensity and the duration of the fast, indicating that the reporter construct is responsive to the expected physiological stimuli (FIG. 4B). The higher variability in the fasting experiment relative to the drug treatment is likely due to variations in food intake and behavior.

Threshold Justification for Primary Screening Assay

The inter-individual variation of ApoB levels in humans was used as a guide to determine a physiologically relevant effect size. A 12% change in ApoB corresponds to a one quintile shift in the population distribution, which was chosen as a good minimum threshold of detection. The percent effect threshold can be set as an arbitrary fraction of the effect size observed in positive control samples. As positive control treatment resulted in approximately a 50% reduction in ApoB levels, a 25% effect size relative to positive control treatment represents about a 12.5% absolute effect size, which was used in the sample size calculation above. Sensitivity to this relatively small effect size leaves a large pool of candidates that can be screened for combinatorial effects.

Assay Tolerance and Reproducibility

Control Parameters:

The Nano-Glo® dual-luciferase assay was developed by the Promega, which has provided detailed literature on the time and temperature dependencies of the assay (see WWW dot PROMEGA dot COM slash RESOURCES slash PROTOCOLS slash TECHNICAL-MANUALS slash 101 slash NANOGLO-DUAL-LUCIFERASE-REPORTER-ASSAY-PROTOCOL slash). The use of an automated assay enables precise control and monitoring of each of these parameters, neither of which are highly variable in the time and temperature ranges to be used in the screen.

Assay Tolerance:

Assay tolerance to DMSO was determined in the concentration range between 0.25 and 3%. In this concentration range, DMSO had no observable effect on signal. However, to avoid potential effects of DMSO alone on zebrafish physiology, the maximal final concentration is 0.1%.

Assay Reproducibility:

There was no significant variation between plates, position within the plate, and experiments performed on different days.

Assay Optimization for High-Throughput Screening

Preliminary results show that the assay is not disruptive to zebrafish metabolism, is responsive to expected physiological and pharmaceutical treatments, is statistically robust, and compatible with existing automated high-throughput screening platforms.

Determine Optimal Time Point for Cessation of Drug Treatment

Zebrafish are nourished by maternal yolk for approximately the first 5 days of development, which provides a period of highly consistent nutrient levels between individual larvae that is amenable to high-throughput screening of metabolic phenotypes. Drug treatment commences a 3 dpf after the liver has differentiated, and the endpoint of drug treatment is optimized empirically by monitoring ApoB production throughout zebrafish development. ApoB levels are measured in developing zebrafish every 12 hours and assess responsiveness to lomitapide treatment to determine the relative rates of ApoB production at each time point. Cessation of drug treatment and reporter quantification just prior to the onset of the fasted state in the pilot screen maximizes the exposure time for each compound and target the full repertoire of larval organs while avoiding the complications of studying fasted zebrafish.

Modify Robotics Footprint to Incorporate Larval Homogenization

The luciferase assay requires larval homogenization prior to reporter quantification. The microplate sonication protocol we have developed provides consistent tissue disruption in low throughput, but needs to be assessed for consistent performance in high-throughput capacity as it is required to process hundreds of plates per day.

Breed and Raise Zebrafish Carrying Homozygous Primary and Counter-Screening Reporters

The ApoB-NanoLuc® luciferase reporter line to be used for the primary screen and the ubi-Fluc line for the counter screening assay have already been developed, but zebrafish homozygous for both reporters are used to ensure true-breeding and produce large numbers of offspring. Heterozygotes for each reporter have been crossed and are being raised to sexual maturity, and a subsequent in-cross of the double-heterozygous fish are used to generate fish that are homozygous for both transgenes. The stock is then expanded and used to generate the large numbers of offspring for the screen.

Screen the JHDL Collection of 3,040 Compounds

Execution of the screen relies on a versatile and rapid reporter-based whole-organism screening platform, termed ARQiv (Automated Reporter Quantification in vivo. Recently combined with a customized robotics work station, this methodology achieves true high-throughput screening rates in vivo (now termed, ARQiv-HTS). Our screen is a modification of this method because the luciferase reporter requires homogenization of the sample and addition of substrate prior to quantification.

Library Selection

The JHDL is comprised of 3,040 compounds, the majority of which are approved for human use and have characterized molecular mechanism(s) of action. This library offers several advantages; first it maximizes likelihood of identifying positive hits from a relatively small compound library as it is validated against all major drug-target classes. Second, the characterized targets enable straightforward identification of genes and pathways involved in the regulation of ApoB. Lastly, hits from this screen can progress rapidly through clinical trials as they are already approved for human use, which has proven to be an effective strategy in drug repurposing.

Quantitative High-Throughput Screening (qHTS)

We employ a quantitative HTS strategy whereby each compound is tested over a dilution series. This approach has been shown to reduce false-positive and false-negative rates as a posteriori validation is provided where graded effects are evident across the titration curve, whereas potentially spurious results are revealed by effects at only a single concentration. Following the recommendations of ARQiv-HTS and power calculations outlined in preliminary data, the screen uses 16 biological replicates and test six serial 1:2 dilutions, testing a concentration range from 0.125 μM to 4 μM. This corresponds to one compound being tested per plate, and the established plate read capacity of 500 plates per day therefore enables testing of 500 compounds in a single day, requiring ˜48,000 larvae.

Large-Scale Egg Production

Customized mass fish breeding chambers are used to produce the large number of transgenic reporter larvae needed for screening. The chambers contain gridded plastic canvases (e.g., Darice, #33900-200) that allow fertilized eggs to descend to a middle chamber where they are collected on a floor of nylon mesh that allows water to flow through the tank while eggs are being collected. Each chamber produces approximately 10,000 eggs per day. Six such tanks can be dedicated to the screen to meet the minimal requirement of 48,000 eggs per day.

Large-Scale Fish Sorting and Dispensing

Complex Object Parametric Analyzer and Sorter (COPAS, Union Biometrica) is used to batch sort viable embryos at 1-2 dpf and to sort viable larvae into 96-well insert plates at later stages (as per prior ARQiv-HTS assays, Wang et al., 2015).

Compound Dispensing and Titration

Hudson Robotics Automated Workstation custom-designed to accommodate ARQiv-based screening and serves all automated handling of compounds, barcoding/reading, and multiwell insert/plate transfers downstream of COPAS dispensing. This includes automated rinse, feed, and anesthetic treatments facilitated by the multiwell inserts.

One Embodiment of a Screen (Outline):

0 dpf: Eggs from are collected and placed in growth media.

1-4 dpf: COPAS batch sorts embryos/larvae each day for viability

4-5 dpf: Viable larvae are treated with test compounds in barcoded plates for 24 hours.

5 dpf: ARQiv-HTS is used to quantify primary and counter screen signal

Confirmation and Retests

All retesting uses the primary screen procedure except that compounds producing maximal effects at the periphery of the dilution series can be retitrated—centering the effective concentration within the dilution series. Consistently significant results indicate confirmed hits and all others are discarded.

One Embodiment of an Orthogonal Secondary Validation Screen Filter for Inhibitors of NanoLuc® Enzyme

Another potential source of false positives are inhibitors of the NanoLuc® enzyme, which can be filtered by exposing the hits to known concentrations of NanoLuc® enzyme and removing leads that attenuate the NanoLuc® luciferase signal directly.

Cross-Screen Toxicity and Teratogenicity—Therapeutic Index

To prioritize compounds as potential leads for testing in mammalian models a ‘therapeutic index’ (TI) is calculated for each confirmed hit. For our purposes, TI is calculated as the ratio between the 50% lethal concentration (LC50) and the minimal effective concentration (LC50/ECmin). Compounds with the highest TI ratios are prioritized as lead candidates and further delineated using assays to assess effects teratogenic effects.

Identify Mechanism of Action and Perform Secondary Screens for Metabolic Benefits

An advantage of the JHDL library is that it is well characterized with regard to targeted signaling pathways. Testing other available small molecule modulators of hit-implicated pathways serves to verify or invalidate specific mechanisms of action in the regulation of ApoB. Additionally, a screen for synergistic combinatorial effects between lead compounds has the potential to greatly increase therapeutic potential over any individual compound.

It is also valuable to evaluate the secondary effects of ApoB-lowering on related metabolic phenotypes. The general approach is to assess the ability of lead compounds to attenuate genetic and dietary-induced metabolic abnormalities using previously developed disease models and fluorescent reporters for insulin signaling, atherosclerotic plaque formation, lipid transport, and inflammation. These high-content secondary screens relies on the VAST automated imaging platform that automatically collects, orients, and images anesthetized zebrafish using a microfluidics array and pattern-matching software. High-content screening with the VAST bioimager is capable of imaging thousands of larvae per week, thus providing ample throughput capacity to function as a secondary screen to HTS.

An additional secondary screen to assess changes in the LDL size distribution relies on gel-based assays rather than high-content imaging. This PAGE protocol above does not require any special instrumentation or expertise, and could easily achieve throughput capacity in excess of 100 samples per day.

Target Identification

Pathway Analysis:

The validated hits from the primary and secondary screening efforts have well-characterized drug targets. Pathway analysis points to common pathways shared between drugs that may have distinct direct targets, but converge on the same downstream effector pathway.

Validation of Pathways and Drug Targets:

The set of predicted targets generated by pathway analysis are validated through treatment with additional compounds known to target the predicted pathway. If the effects of treatment with the additional compounds are similar to those of treatment with the lead compound, it serves as strong evidence that the predicted pathway is the true drug target resulting in the change in ApoB levels.

Combinatorial Screen

Lead compounds are used in combination at each of their respective maximum-effect concentrations and assayed for effectiveness in the primary screening assay. Combinations showing synergistic effects are included as an additional combination therapy for secondary screening.

Secondary Screens for Related Metabolic Phenotypes

Dietary and Genetic Disease Models:

The zebrafish is a powerful established model of metabolic disease. Feeding zebrafish a high-fat diet has been shown to induce obesity and atherosclerotic plaques, and genetic models of diabetes and inflammation have already been developed.

Drug Treatment:

Fish of the appropriate transgenic line are treated continuously from 5 to 12 dpf with the drug concentration that achieved the maximal effect size in the primary screen.

Automated Bioimaging:

Following each of the experimental treatments outlined below as secondary screens, larvae are automatically collected and imaged using the VAST bioimaging platform (see WWW dot UNION BIO dot COM slash VAST slash).

Unbiased Statistical Image Analysis:

We have pioneered “Finotyper”—a statistical analysis pipeline specifically designed to analyze fluorescent images of zebrafish. The program uses principal component analysis to cluster images into statistically significant groups based on the intensity and distribution of fluorescent signal, which can be used to test for differences between control and treatment groups in the high-content secondary imaging screens (FIGS. 5A-5B). Additionally, the program highlights the regions of the image that are responsible for the differential clustering, facilitating identification of the regions effected by treatment.

Secondary Screen for Changes in Lipid Transport

Fluorescent lipid analogs introduced into the diet are absorbed and processed very similarly to natural lipids, and provide a fluorescent readout of lipid accumulation in various tissues. Lipids are absorbed by the intestine and secreted into the bloodstream where they are absorbed by diverse peripheral tissues but accumulate primarily in the liver and gallbladder. A single fluorescent lipid pulse is included in the meal of wild-type larval zebrafish pretreated with lead compounds of interest or vehicle control. Following termination of the feed and a 4-hour chase period, fluorescent lipid localization is recorded by the VAST bioimager and analyzed for differences in treatment groups using finotyper (FIG. 5C).

Secondary Screen for Changes in Insulin Signaling

Phosphoenolpyruvate carboxylase (pepck) is a hepatic enzyme essential for gluconeogenesis, and is therefore upregulated in the fasting state as well as states of insulin resistance. A fluorescent and bioluminescent reporter of phosphoenolpyruvate carboxykinase (pck1) has already been developed as a means of screening for compounds that improve insulin signaling in larval zebrafish. Changes in reporter gene expression following drug treatment are collected and analyzed with the VAST-Finotyper pipeline.

Secondary Screen for Changes in Atherogenesis

Zebrafish is an established model of atherogenic plaque formation with multiple approaches to assess plaque formation in vivo. One approach involves feeding of fluorescent lipid analogs, which have been shown to accumulate in perivascular punctae in response to high-fat diet. Additionally, a transgenic zebrafish line has been developed that expresses a fluorescent antibody to oxidized LDL on an inducible heat-shock promoter (hsp-70:scaOxLDL). High fat feeding promotes formation of plaques rich in oxidized LDL that can be visualized following acute heat shock. Assessment of lead compounds for atheroprotective effects in the context of high-fat diet can be useful in prioritizing hits for clinical development, and correlation of these data with lipoprotein size distribution provides further insight into the relationship between LDL size distribution and atherogenic potential. Changes in antibody localization are assessed using the VAST-Finotyper pipeline.

Secondary Screen for Changes in Inflammation

Transgenic reporters of inflammation have also been developed for the larval zebrafish. Generation of hydrogen peroxide is a hallmark of inflammation, and the HyPer sensor relays the increase in hydrogen peroxide production into a fluorescent readout through fusion of a modified yellow fluorescent protein (cpYFP) to a split version of the prokaryotic hydrogen peroxide sensing protein OxyR. Ratiometric measurement of the native and cross-linked fluorescent signals from the HyPer sensor provides a localized readout of inflammation. The HyPer sensor fish are exposed to a high fat diet including treatment with lead compounds of interest or carrier control and assessed for changes in the degree of inflammation induced as well as the tissue localization using the VAST-Finotyper pipeline.

Secondary Screen for Changes in Lipoprotein Size Distribution

The size distribution of atherogenic lipoprotein particles is a potentially important risk factor for CVD, as smaller LDL particles have been shown to have numerous atherogenic properties, including resistance to clearance by the LDL-receptor, higher propensity to enter the vascular intima, and higher susceptibility to oxidation. The in-gel luciferase assay developed in our lab provides the opportunity to rapidly and sensitively detect changes in lipoprotein size distribution using individual larval zebrafish. Following a week of high-fat diet with or without treatment of test compound, larval zebrafish are homogenized and lipoproteins are separated on a 3% native PAGE gel and imaged using the in-gel luciferase reporter. This enables rapid assessment of lead compounds on effects of the potentially clinically important lipoprotein size distribution phenotype. As a proof of principle, this method was used to assess the change in lipoprotein size distribution in response to acute lomitapide treatment (FIGS. 6A-6B). Lomitapide treatment caused a significant reduction in ApoB levels, and also induced the formation of very small lipoprotein particles.

The present invention has developed a high-throughput phenotypic screen for modulators of ApoB in larval zebrafish and presents a remarkable opportunity to perform unbiased investigation of drug targets in every cell and tissue type while compensating for all the complex signaling and feedback typical of vertebrate metabolism, thus maximizing the likelihood of identifying viable pre-therapeutic leads. This approach can have a resounding impact on the burden of metabolic disease by providing numerous resources to the research and clinical communities, including: (1) an in vivo HTS assay for modulators of ApoB, (2) identification of novel genetic regulators of ApoB, (3) a collection of probes for studying the regulation of ApoB, (4) improved methods for automated image analysis, (5) extensive characterization of relationships between ApoB and related metabolic disease abnormalities, and (6) promising pre-therapeutic leads for treatment of several metabolic diseases.

Apolipoprotein-B (ApoB) is a remarkable biomarker for metabolic health, as it integrates many of the central risk factors for metabolic disease into a single phenotypic readout. Insulin resistance and hepatic triglyceride accumulation both increase ApoB levels, and elevated ApoB in turn promotes ER stress, atherogenesis, and chronic inflammation. ApoB levels are consequently one of the strongest predictors of diabetes, metabolic syndrome, and cardiovascular disease in humans. Compounds that lower ApoB levels could therefore engender numerous metabolic benefits, including improvement of insulin sensitivity, reduction of hepatic triglyceride content, uncoupling of metabolic dysfunction from diabetes and cardiovascular disease risk, amelioration of ER-stress and chronic inflammation, or some combination of these actions. In order to identify such compounds, an in vivo, high-throughput screen (HTS) for ApoB-lowering compounds using transgenic zebrafish larvae carrying an optical reporter of ApoB is carried out.

The larval zebrafish is ideal for our study as it (i) recapitulates all major aspects of vertebrate metabolism in a small, rapidly developing organism, (ii) is the only vertebrate system conducive to HTS, and (iii) has a proven to be a powerful model for drug discovery owing to remarkably conserved physiology and pharmacology with humans. We have used state-of-the-art genome engineering techniques to create transgenic zebrafish carrying a luciferase reporter fused to ApoB (ApoB-NanoLuc® luciferase), which enable rapid, accurate, and highly sensitive quantification of ApoB levels at high-throughput rates using an automated whole-organism screening platform developed for zebrafish chemical biology assays. The primary screen uses the published Automated Reporter Quantification in vivo coupled to High-Throughput Screening (ARQiv-HTS) to identify novel modulators of ApoB in live zebrafish using the ApoB-NanoLuc® luciferase reporter. The assay is a titration-based, in vivo, phenotypic screen that includes an internal counterscreen, secondary validation, and orthogonal screening, thus encompassing many of the best practices in the field and maximizing the probability of success. A pilot screen is performed using a ˜3,000 compound library, and post hoc statistics and subsampling is used to optimize the layout and screen an additional 30,000 compounds.

ApoB is an Underappreciated Therapeutic Target for Treatment of Metabolic Disease

Pharmaceutical treatments for metabolic disease have historically focused on regulating serum metabolite levels, such as normalizing glucose levels in the context of diabetes and lowering serum cholesterol levels to prevent cardiovascular disease. However, these treatments allow many intracellular and systemic metabolic abnormalities to persist. ApoB is both a useful biomarker and a causative agent for many of the untreated aspects of metabolic disease, and is therefore uniquely powerful therapeutic target, yet drug development efforts have made little progress in this area. Here, we focus on ApoB as a risk factor and use an unbiased in vivo drug discovery approach enabling the identification of compounds that treat the causal factors underlying metabolic disease rather than simply restore serum metabolite homeostasis.

Larval Zebrafish Enable Unbiased In Vivo Screening

ApoB is regulated by a complex homeostatic network involving nutritive, transcriptional, hormonal, and cell-signaling inputs from numerous cell types across almost every major organ. Previous screening efforts in cell culture (1) have been restricted to screening for modulators of ApoB production or uptake in a single cell type, and therefore cannot recapitulate the many intravascular processing, metabolite, endocrine, and neuronal factors contributing to ApoB regulation in vivo. Screening in live vertebrates affords the opportunity for completely unbiased screening that is sensitive to ApoB modulators in every cell and tissue type, but screening in mammalian systems has very limited throughput capacity. The larval zebrafish has recently emerged as the premiere model system for HTS in vivo, as it is easy to produce a large number of rapidly developing organisms that can be maintained and exposed to test compounds in 96-well plate format. Importantly, phenotypic screens such as ours have had remarkably high success rates in first-in-class compound discovery in recent decades (2).

Larval Zebrafish Recapitulate all Major Aspects of ApoB Homeostasis

Larval zebrafish show remarkable conservation of pathways related to ApoB metabolism compared to humans, further justifying their use in our screen. By 5 days post-fertilization, zebrafish larvae have developed all major digestive organs, and therefore possess all the cell and tissue types that contribute to ApoB regulation. The Farber lab has pioneered the use of zebrafish to study apolipoprotein biology (3), and shown that although there are multiple duplications of the ApoB gene in zebrafish, a single isoform (ApoBb.1) is responsible for producing over 95% of the ApoB mRNA and protein from both the liver and intestine. We focus on this single (dominant) isoform. The ApoB coding sequence is well conserved when compared to humans (4, 5), with particularly high sequence conservation in all the essential functional domains such as the MTP-interacting domain, the LDL-receptor binding site, a proline-rich region, and the conserved residue that causes familial hypercholesterolemia when mutated in humans (5-7). Although two of the proline-rich repeats are absent in zebrafish ApoB, these domains are of unknown function and are thought to have arisen from partial gene-duplication and likely have little functional importance (7). In humans, a full-length ApoB-100 protein is expressed in the liver (ApoB-100), but the transcript is post-transcriptionally modified in the intestine to form a truncated (ApoB-48) isoform. Zebrafish have neither the cytosine deaminase enzyme (APOBEC1) nor the conserved modification site (cytosine 6,666) that mediate post-transcriptional processing in mammals (8), indicating that zebrafish produce exclusively full-length (ApoB-100-like) ApoB. The physiological relevance of the truncated ApoB-48 is not well defined, but has been suggested to permit more efficient production of triglyceride-rich lipoproteins under conditions of excess fat intake (9). Importantly, as beta-lipoproteins of both liver (apoB-100) and intestinal (ApoB-48) origin are risk factors for cardiometabolic disease (10), the lack of a truncated isoform in zebrafish allows us to tag all ApoB proteins with a single carboxy-terminal tag. This single reporter fusion protein is therefore sensitive to modulators of both gut and liver derived beta-lipoproteins, and is not susceptible to off-target effects of deamination inhibitors that might confound results in other systems. In addition to high conservation of the functional domains of ApoB, zebrafish also have conserved orthologs of the majority of genes involved in ApoB production, processing, and uptake, and mutations in these genes recapitulate the phenotypes seen in corresponding human disease (11, 12).

Zebrafish Drug Screening is a Powerful Tool for Human Drug Discovery

Zebrafish are not only amenable to high-throughput screening, but compounds discovered in zebrafish have identified promising leads for the development of human drugs. Several drugs discovered in zebrafish screens are in various stages of clinical development, including Prohema (phase II) (13), COX inhibitors (Phase I) (14), Dexamethasone (Phase I) (15), Dorsomorphin (preclinical development) (16), and PROTO-1 (lead optimization) (17). While no drugs initially discovered in zebrafish are yet on the market, this is attributable to the relative infancy of drug screening in zebrafish (over 95% of published zebrafish drug screens took place after 2007) (18) and the long process from drug discovery to market (averaging about 12 years) (19). There are also plentiful examples of human therapeutics showing similar activity in zebrafish, including anti-diabetic, cardiovascular, anti-angiogenic, anti-cancer (20), cardiotoxic, atheroprotective (21), and psychoactive compounds (22, 23). Additionally, zebrafish are highly genetically conserved to humans as 82% of the proteins implicated in human disease have annotated orthologs in zebrafish (24). The high genetic and pharmacological conservation between humans and zebrafish and the success of previous screening efforts provide strong justification for the continued use of zebrafish as a tool for human drug discovery.

Luciferase-Tagged ApoB Permits Sensitive Characterization of Lipoprotein Phenotypes

To sensitively detect ApoB-levels in individual zebrafish, we have used TALEN-mediated precise genome engineering to introduce the NanoLuc® luciferase reporter as a carboxy-terminal tag on ApoB. NanoLuc® luciferase is an engineered luciferase reporter that is half the size (14 kDa) and ˜100-fold brighter than firefly luciferase, shows essentially no background signal, and allows for independent parallel quantification of a second reporter (firefly luciferase) using a dual-luciferase assay, thus engendering unparalleled sensitivity to our reporter assays (25). We have collected substantial preliminary data to validate that the NanoLuc® luciferase tag does not disrupt ApoB function. We demonstrated that NanoLuc® luciferase signal accumulates in the expected lipoprotein density fractions, that the lipoproteins present in each fraction are the expected sizes (20-60 nm), and that the particles show the expected electrophoretic mobility on a 3% polyacrylamide gel.

We then set out to verify that specific dietary, genetic, and pharmacological manipulations known to modulate ApoB in humans had similar effects in zebrafish. Specifically, a high-fat meal increases lipid availability and concomitantly raises ApoB levels in humans, and the ApoB-NanoLuc® luciferase reporter shows the expected induction in response to high-fat feeding. Humans lacking Apolipoprotein-CII (ApoC2) have profoundly increased ApoB levels (hyperlipoproteinemia type IB) as a result of defects in processing and turnover of ApoB-containing lipoproteins. We have used CRISPR to target the apoc2 gene in zebrafish, and shown that zebrafish show the expected hallmarks of hyperlipoproteinemia type IB including elevated ApoB levels and accumulation of large lipoprotein particles. Finally, pharmaceutical treatment using lomitapide (an MTP inhibitor) results in significant reduction of ApoB levels in humans, and significantly lowered ApoB-NanoLuc® luciferase levels in transgenic larvae as would be expected. Finally, we performed a developmental time-course to determine the optimal treatment window for compound screening. Larvae are nourished by maternal yolk at this stage, which is absorbed and packaged into beta-lipoproteins at a stereotypic rate throughout development and provides predictable reference levels to identify modulators of ApoB A treatment window from 3-5 dpf was selected as the liver is fully differentiated and ApoB is being actively produced and turned over at these stages. In conclusion, in every respect the ApoB-NanoLuc® luciferase reporter has behaved as expected based on our understanding of ApoB biology in humans.

Overview

There are detailed protocols for the development and execution of a high-throughput screen using live vertebrate zebrafish (26, 27). The high-throughput screen uses transgenic zebrafish that carry both an engineered luciferase reporter (NanoLuc® luciferase) fused to ApoB which serves as the primary readout, and a second ubiquitously expressed firefly luciferase reporter that is used as an internal control. Transgenic larvae are raised to 3 days post-fertilization (dpf) and automatically sorted and dispensed into 96-well plates containing various dilutions of test compound. Following 48-hour incubation with test and control compounds, the zebrafish are homogenized and assayed sequentially for Firefly and NanoLuc® luciferase signal using the automated robotics platform and plate reader. Hit-calling software is used to select putative hits, which are validated through repetition and orthogonal screening. The first iteration of the screen uses a modest-sized library (3,000 compounds), and empirical determination of the optimal sample size and number of titration points required for screening enables an increase in throughput for the following iteration of the screen, which screens a 30,000 compound library.

High-Throughput Screening Platform

The primary screen uses the ARQiv-HTS screening platform (automated reporter quantification in vivo coupled to high-throughput screening robotics) (27) to identify compounds that modulate ApoB levels in live zebrafish using the ApoB-NanoLuc® luciferase reporter line. The screening platform is slightly adapted to quantify chemiluminescent (rather than fluorescent) reporters by including a homogenization step and the addition of luciferase substrates.

Reporter Assay Performance

Chemiluminescent reporters offer a highly sensitive, quantitative signal with unparalleled signal to noise ratios, making them ideal for HTS applications. This assay uses the transgenic line homozygous for the ApoB-NanoLuc® luciferase reporter described above as a primary readout of ApoB levels. A standard metric for HTS assay quality is the Z′-factor (Z′), which has an arbitrary threshold of 0.5 for an HTS-ready assay (28). Z′ calculations for this assay reveal its excellent performance (>0.5) based on genetic positive and negative controls (FIG. 7A, reflecting very high signal to background ratios), but poorer performance based on lomitapide, the best pharmacological positive control available (FIG. 7A). Poorer assay performance in the latter experiment is a result of both the inherent variability of live vertebrate systems, as well as the lack of availability of a sufficiently potent ApoB-lowering therapy to use as a positive control. To contextualize these calculations, humans treated with the strongest ApoB-lowering therapies available (lomitapide and a PCSK9 antibody) show similar magnitudes of reduction in ApoB with higher levels of variability (29, 30) (FIG. 7B), and have poorer Z′-factors (−0.63 and −1.02). The lower variability in the zebrafish model is likely due to the highly consistent diet of maternal yolk used to nourish zebrafish during the screen. The inability of the strongest pharmaceuticals available to achieve a Z′ greater than 0.5 in humans or zebrafish indicates that this may not be a realistic threshold to screen for modulators of ApoB in live vertebrates.

A previous HTS in live zebrafish was also unable to reach the Z′ threshold of 0.5 (FIG. 7C, note screen was for compounds that increase signal), but adopted an alternative statistical framework based on the strictly standardized mean difference (SSMD) statistic, which (i) is frequently used for in vivo screening (such as RNAi-screens), (ii) accounts for the inherent variability in live vertebrate systems, and (iii) is better suited to assays where an extremely strong positive control is not available or appropriate (31, 32). This screen was able to successfully identify over 200 hits from a library of 3,000 compounds, thus providing precedent that the SSMD calculations are a reasonable substitute for the Z′-factor for in vivo HTS (26). The assay described here greatly outperforms the assay used in the previously published screen in zebrafish mentioned above (26) as measured by both the Z′-factor and power calculations.

In summary, the screening assay shows excellent statistical performance based on genetic controls (Z′>0.5), better performance than expected using pharmaceutical controls as compared to ApoB-lowering treatments in humans, and also shows superior assay quality to the previous successful HTS in zebrafish, suggesting a high probability of success in the primary screen.

Sample Size, Error Rate, and Hit Selection Calculations

Statistical resampling of positive control treatments can be used to estimate the appropriate sample size for HTS assays given type I and type II error rate thresholds and a desired cutoff for effect size (27). Assuming a type II error rate of 0.01 and setting the hit selection cutoff to 50% the magnitude of positive control treatment generates a log-linear relationship between sample size and type I error rate. A sample size of 9 corresponding to an error rate of 0.01, suggesting that a relatively small number of biological replicates can be used to perform a robust primary screen (previous assays required a sample size of 16). Besides serving as a metric for assay quality, the SSMD score can also be used to determine hit cutoff criteria. This statistical test compares the averages between the negative control and each experimental sample, and then adjusts this difference based on the median absolute deviation (MAD) of each sample, such that samples with high variability are penalized and produce a poor SSMD score even if there is a large difference between the means. In positive control treatments, lomitapide produced an SSMD score of −4.5, reflecting a very strong control assay. The scaling factor formula can be used to determine the SSMD cutoff based on the desired magnitude relative to the positive control. A compound with a 50% effect size relative to MTP was chosen as a physiologically relevant effect size, which would produce an SSMD score of −2.7.

Pilot Screening Protocol

Plate Layout:

The plate layout is similar to that in the ARQiv-HTS protocol (27), in that a positive and negative control plate precedes each block of 10 experimental plates, which are used to ensure that no position effects occur in the screening process as well as collect positive and negative control values for SSMD calculation that temporally bracket the experimental samples and account for signal variation across time. Additionally, each plate includes eight positive and eight negative control samples to account for variability between plates. Test compounds are arrayed in a set of 16 biological replicates and a 5-point titration series of 2-fold dilutions to generate a very high quality data set in the pilot screen. It should be noted that while power calculations suggest that nine biological replicates would be sufficient to detect the desired effect size, nine replicates at five concentrations still occupies more than half of the available wells, precluding screening of two compounds per plate. By choosing to generate a high-quality dataset (with 16 biological replicates) in the pilot screen (one compound per plate), it is possible to optimize the plate layout for the subsequent larger screen and detect compounds with small effect sizes with low error rates. The large-scale screen adopts a different plate layout as suggested by subsampling analysis, with the goal of screening of 2-4 compounds per plate.

Library Selection:

The Johns Hopkins Drug Library (JHDL) is comprised of 3,040 compounds, the majority of which are approved for human use and have characterized molecular mechanism(s) of action (26, 33). This library offers several advantages for the pilot screen; first it maximizes likelihood of identifying positive hits from a relatively small compound library as it is validated against all major drug-target classes. Second, the characterized targets permit rapid identification of putative drug targets through pathway analysis. Lastly, hits from this screen can progress rapidly through clinical trials as they are already approved for human use (34-36). The subsequent large-scale screen uses the 30,000 compound CombiSet library (Chembridge) to maximize diversity within an achievable library size.

Internal Control Dual Luciferase Assay:

The NanoLuc® luciferase reporter uses a different substrate than firefly luciferase, allowing a second luciferase reporter to be measured in parallel as an internal control. We chose to use a transgenic line expressing Firefly luciferase (Fluc) reporter in every cell (driven by the ubiquitin promoter) for this purpose (37). By monitoring levels of firefly luciferase, the screening assay is able to detect if any treatments result in developmental delay, cytotoxicity/death, or general disruptions in transcription, translation, or protein turnover/stability. The internal control line was bred with the ApoB-NanoLuc® luciferase reporter line, and the progeny were incrossed and to create a true-breeding stock homozygous for both reporters.

Screening Protocol:

The protocol has been designed such that up to two simultaneous iterations for the screening protocol can be performed per week.

Day 1: Embryo production and collection—zebrafish homozygous for the ApoB-NanoLuc® luciferase and ubi:Fluc reporters are induced to spawn using custom mass-breeding chambers. Eggs are collected hourly from 9 AM to 2 PM. Embryos are maintained in petri dishes at a density of 5 fish/mL of embryo medium (E3). Day 2: Raising embryos—Embryos are maintained in E3 (1 dpf). Day 3: Raising embryos and preparing drug plates—Embryos are maintained in embryo medium (2 dpf). The titration protocol is initiated that dilutes and arrays drugs into 96-well barcoded flat-bottom skirted plates following the plate layout using the Solo automated pippettor. Each well contains 100 μL of E3 with twice the final concentration of compound and vehicle specified in the plate layout. Subsequent dispensing of the zebrafish larvae dilutes this stock to the final treatment concentration. Day 4: Treatment initiation—The COPAS begins automatically dispensing larval zebrafish into individual wells of each 96 well plate at 3 dpf. Filled plates are lidded and transferred to the housing racks for incubation. Day 5: Incubation—Larvae continue incubation with test compounds. Day 6: Quantification—Larvae are transferred to the continuously operating plate-horn sonicator using the plate crane and processed, and 40 μL of the homogenate is transferred to a read plate and quantified for firefly and NanoLuc® luciferase signal in the Tecan M1000 plate reader by serial addition of their respective substrates. Preliminary Screening Data

We have validated the screening assay protocol on a small collection of compounds implicated in the regulation of lipid and lipoprotein metabolism (FIG. 8A). Consistent with our understanding that very few known compounds modulate ApoB metabolism, none of the compounds tested resulted in a significant change in ApoB except for the positive control compound, lomitapide. The screening assay includes firefly luciferase as an internal control reporter to detect off target or cytotoxic effects, and the majority of compounds tested resulted in almost no effect on firefly luciferase levels, as evidenced by the tight distribution of SSMD scores for firefly luciferase around zero. Cutoff values for firefly luciferase were set to positive (increase in signal) and negative (decrease in signal) 2.4, corresponding to 6 standard deviations away from the average SSMD score. Any SSMD scores outside of this range (denoted by dashed gray lines) is discarded due to off-target effects. Several concentrations of test compounds were lethal to zebrafish larvae, and as expected returned very strong negative SSMD scores (<−10) that clearly signal for exclusion from analysis.

ApoB-NanoLuc® luciferase is the primary readout and the hit selection cutoff is set to −2.7 (FIG. 8B, dashed). Positive control treatment with lomitapide resulted in a dose-dependent decrease in NanoLuc® luciferase levels that exceeds threshold cutoff requirements without affecting the internal control reporter (FIG. 8B).

Results from the preliminary screen support that (i) the screening process is robust and free of false positives, (ii) cytotoxic/lethal compounds can be readily detected and discarded, and (iii) the positive control treatment results in strong, dose-responsive effects that meet all hit selection criteria.

Hit Validation and Orthogonal Screening

Hits identified are re-tested using the primary screen procedure except that compounds producing maximal effects at the periphery of the dilution series are re-titrated to center the effective concentration within the dilution series. Consistently significant results indicates confirmed hits and all others are discarded. Small molecule inhibitors of NanoLuc® enzyme activity serve as a potential source of false-positives, which are filtered out by treating known concentrations of NanoLuc® enzyme with hits from the screen and removing leads that attenuate NanoLuc® luciferase signal directly. A sandwich-ELISA based method specific for fish beta-lipoproteins (MyBioSource, MBS008940) serve as an orthogonal test to validate that hits attenuate ApoB levels using a measurement method independent of the NanoLuc® luciferase reporter.

Empirical Optimization of Plate Layout and Execution of Optimized Large-Scale Screen

The pilot screen is performed using a sample size of 16 biological replicates across a 5-point titration series of 2-fold dilutions, as power calculations suggest that this produces a very high-quality dataset. This dataset is used to empirically determine the effects of sample size and number of titration points on error rates by repeatedly sampling subsets of the data (bootstrapping). An optimized screen is then executed using the refined plate layout that optimizes throughput capacity while keeping error rates low based on empirical data. This optimized screen can execute two iterations of the screen per week, and is projected to run approximately twice as long (˜2 years), permitting a four-fold increase in library size without modification of the plate layout. If the layout can be modified to include 2-4 drugs per plate, this results in an 8-16 fold increase in overall throughput capacity. The CombiSet library (Chembridge) is ideal for our screen as it maximizes library diversity within a manageable library size (30,000 compounds).

Additional Assays to Characterize the Size and Localization of ApoB-Containing Particles

In addition to its utility as a high-throughput compatible readout for measuring levels of Apolipoprotein B, the described ApoB-NanoLuc® luciferase reporter can also be used to measure the size distribution and localization of ApoB-containing particles in zebrafish larvae. Lipoproteins separated using polyacrylamide gel electrophoresis can be sensitively detected in individual larval homogenates by immersing the gel in the NanoLuc® luciferase substrate. The gel shown confirms that, as expected, fish lacking the ApoC2 gene are unable to lipolyze large lipoprotein particles: chylomicrons (CM) and very-low density lipoprotein (VLDL) particles into their smaller counterparts, intermediate density lipoproteins (IDL) and low-density lipoproteins (LDL) (FIG. 9A). Conversely, fish treated with the MTP inhibitor lomitapide are unable to create large lipoprotein particles (FIG. 9A). Additionally, immersion of transgenic larvae in the luciferase substrate enables visualization of the localization of ApoB throughout the organism (FIG. 9B).

The pilot screen is performed using a sample size of 16 biological replicates across a 5-point titration series of 2-fold dilutions, as power calculations suggest that this produces a very high-quality dataset. This dataset is used to empirically determine the effects of sample size and number of titration points on error rates by repeatedly sampling subsets of the data (bootstrapping). An optimized screen is then executed using the refined plate layout that optimizes throughput capacity while keeping error rates low based on empirical data. This optimized screen can execute two iterations of the screen per week, and is projected to run approximately twice as long (˜2 years), permitting a four-fold increase in library size without modification of the plate layout. If the layout can be modified to include 2-4 drugs per plate, this results in an 8-16 fold increase in overall throughput capacity. The CombiSet library (Chembridge) is ideal for our screen as it maximizes library diversity within a manageable library size (30,000 compounds).

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All references cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.

A term such as “a” or “an” or “the” in the context of describing the invention (especially in the context of the claims) is to be construed as including both the singular (“one”) and the plural (“more than one”), unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The examples and exemplary language (e.g., “such as”) provided herein, are intended merely to better understand the invention and are not limitations on the scope of the invention unless the exemplified element is recited in the claim. No language in the specification should be construed as indicating that a non-claimed element is essential to the practice of the invention.

Specific embodiments are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those specific embodiments may become apparent to a person skilled in the art upon reading the foregoing description. Thus, one of skill in the art could practice the invention using such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this description includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context. 

The invention claimed is:
 1. A method for high-throughput, in vivo screening; the method comprising: (a) applying an agent to a zebrafish expressing an Apoliprotein B (ApoB)-reporter fusion protein; (b) monitoring the reporter activity; (c) comparing the reporter activity to a second reporter activity of a reference; and (d) identifying a modulator of ApoB.
 2. The method according to claim 1, wherein the ApoB-reporter fusion protein is expressed from an ApoBb.1 locus-reporter gene.
 3. The method according to claim 1, wherein the reporter is a luciferase.
 4. The method according to claim 1, wherein identifying a modulator of ApoB enhancing ApoB expression occurs when the reporter activity is greater than the second reporter activity of the reference.
 5. The method according to claim 1, wherein identifying a modulator of ApoB inhibiting expression of ApoB occurs when the reporter activity is less than the second reporter activity of the reference.
 6. The method according to claim 1, wherein the agent is a chemical.
 7. The method according to claim 1, wherein the reporter is a luciferase and the ApoB-reporter fusion protein is expressed from a DNA sequence of SEQ ID NO:
 3. 8. The method according to claim 1, wherein the ApoB-reporter fusion protein has a protein sequence of SEQ ID NO:
 2. 9. A zebrafish comprising an ApoBb.1 locus-reporter fusion gene.
 10. The zebrafish as in claim 9, wherein the ApoBb.1 locus-reporter fusion gene has a DNA sequence of SEQ ID NO:
 3. 11. The zebrafish as in claim 9 further comprising a genomic ubiquitous promoter driving expression of a firefly luciferase gene.
 12. The zebrafish as in claim 9 further comprising a genomic mCherry fluorescent reporter.
 13. A method for high-throughput, in vivo screening to identify a modulator of ApoB; the method comprising: (a) applying agents to zebrafish larvae expressing an ApoB-reporter fusion protein gene and a second reporter protein; (b) monitoring the ApoB-reporter fusion protein activity and the second reporter protein activity; and (c) comparing the reporter activities to the third reporter activities of a reference.
 14. The method according to claim 13, wherein the ApoB-reporter fusion protein is an ApoB-luciferase fusion protein.
 15. The method according to claim 14, wherein the ApoB-mutant luciferase fusion protein is expressed from an ApoBb.1 locus-mutant luciferase gene fusion.
 16. The method according to claim 13, wherein the monitoring comprises sonicating the zebrafish larvae and then measuring the ApoB-fusion protein activity and the second reporter protein activity by a high content screening (HCS) microscopy platform.
 17. The method according to claim 13, wherein the second reporter is firefly luciferase. 